1. Field of the Invention
The present invention relates to a method for producing a multijunction solar cell.
2. Description of the Related Art
Conventionally, a single-junction solar cell comprising a single pn junction has been known. However, it is known that the power generation efficiency of the single-junction solar cell is only about 30% under terrestrial sunlight irradiation conditions irrespective of kind of semiconductor materials used, because the power generation efficiency has a theoretical limit determined by the band gap Eg of a semiconductor material used as a raw material.
Thus, in order to obtain power generation efficiency higher than that of the single-junction solar cell, there is proposed a multijunction solar cell formed by laminating a plurality of solar cells in which each cell has a pn junction. As the simplest of the multijunction solar cells, there is known a two-junction solar cell 11 as shown in FIG. 5 (refer to Japanese Patent Laid-Open No. 9-64386).
The two-junction solar cell 11 comprises a back electrode 12, a bottom cell 13 laminated by epitaxial growth thereon, a top cell 14 laminated by epitaxial growth on the bottom cell 13, and a surface electrode 15 laminated on the top cell 14. In the two-junction solar cell 11, the bottom cell 13 comprises a pn junction of GaAs, and the top cell 14 comprises a pn junction of InGaP.
The basic principle of the multijunction solar cell is to expand the available region of the sunlight spectrum by joining semiconductors having different band gaps, and it is known that efficiency will increase when the number of junctions is increased. For example, a four-junction solar cell has been simulated, and a highly efficient solar cell is expected.
However, a combination of semiconductors having different band gaps generally results in a combination of semiconductors having lattice constants which are different to each other. Therefore, when semiconductors having largely different lattice constants to each other are intended to be joined, a defect will occur at the interface. This poses a problem that the efficiency cannot be improved. This problem is due to the fact that since a large number of dangling bonds present in the defect cause the recombination of excitons excited by sunlight, it is impossible to take out them as electric power.
As a result, the semiconductors which constitute the multijunction solar cell are limited to a combination of materials having lattice constants similar to each other (a combination in which so called lattice matching is achieved) while these materials have band gaps different to each other. When matching with the sunlight spectrum is added to the above combination, the semiconductors will be limited to a combination for constituting a three-junction solar cell such as a combination of InGaP, GaAs, and Ge, or a combination of InGaP, InGaAs, and Ge.
GaInNAs is the only material that attracts attention as a material having a band gap of about 1 eV while maintaining lattice matching with Ge and GaAs. Research of a four-junction solar cell by combining InGaP, InGaAs, GaInNAs, and Ge has been continued (Refer to J. F. Geisz, D. J. Friedman, C. Kramer, A. Kibbler, and S. R. Kurtz, “New Materials for Future Generations of III-V Solar Cells”, NREL/CP-520-25631, National Renewable Energy Laboratory, December 1998).
However, GaInNAs has the disadvantage of difficulty in crystal growth for achieving a four-junction solar cell due to the occurrence of phase separation and defects.
Moreover, the performance of a three-junction solar cell has been evaluated by a device of 4 cm2 or less, and it is known that the efficiency is reduced as the area of the device increases (Refer to Akira Ohmae, Yukiko Shimizu, and Yoshitaka Okada, “GaInNAs for Multi-Junction Tandem Solar Cells”, Photovoltaic Energy Conversion, 2003, Proceedings of 3rd World conference on Volume 2, 12-16 May 2003). The reduction of the efficiency is assumed to be due to the occurrence of defects near the junction interface even in a two-junction solar cell or a three-junction solar cell because the semiconductors have lattice constants different to each other.
Therefore, the multijunction solar cell has the disadvantage of difficulty in increasing the area of the device.